Before discussing in detail the present invention, reference will be made to the general aspect of a cathode ray tube. As shown in FIG. 1, the cathode-ray tube generally includes an envelope or a body made of a glass material, which comprises a neck portion 1, a conical portion 2 integrated thereto, and a display portion 3.
The conical portion 2 is provided on the outer surface with an electrically conductive, outer coating film 6 and on the inner surface with an electrically conductive, inner coating film 5, which is contiguous to a high-voltage conductor 4 adapted to be applied with a high-voltage potential. The outer coating film is, on the other hand, at earth potential.
Such coating films may usually be obtained from a coating material prepared by dispersing a metal oxide such as titanium oxide and graphite in an aqueous solution of an alkali metal silicate such as water glass.
An electron gun 7 is accommodated within the neck portion 7 with contactor 8 being in contact with the inner coating film 5. Reference numeral 9 stands for a shadow mask.
Incidentally, it is substantially unavoidable that a slight amount of the inner coating film peels off in the form of fine pieces during the production process and later handling of cathode-ray tubes, e.g., due to friction with the contactor 8 at the time of mounting of the electron gun. Since a voltage of ten plus several KV or higher is usually applied on the electron gun while the cathode-ray tube works in good order, a short circuit takes place between the electrodes of the electron gun 7 by those fine pieces, thereby giving rise to a spark current reaching as high as 500 .ANG..
Such a current flows in an electronic circuit connected to the cathode-ray tube by way of induction coupling or capacity coupling, thus giving damage to semiconductor parts having a low resistance to voltage, insulating parts, etc., or the electrodes per se of the electron gun. Practically, a spark current exceeding 200 .ANG. is taken as being hihgly dangerous.
To avoid such a short-circuit accident, it has been proposed to increase the resistance of the coating film to a high value of 1M.OMEGA. to 100M.OMEGA., as disclosed in U.S. Pat. Nos. 2,545,120 and 2,829,292. However, such a proposal is found not to be suitable for currently available production systems. This is because the production process involves a step for applying a voltage higher than designed on the electrodes for sparking at an initial stage. If the resistance of the coating film is higher than 100 K.OMEGA., then breakage of the insulation applied on the cathode-ray tube takes place, so that small holes are instantaneously formed in the wall thereof.
There have also been various proposals directed to the limitation of specific resistance values of the coating film, i.e., static and dynamic resistance values (see Japanese Patent Publication No. 58-32735), the method of preparing resistant coating materials (see Japanese Patent Publication No. 53-9400), etc.
However, it has been found that such proposals do not give satisfactory results, or involve some practical difficulties.
As a result of intensive studies made of a number of factors attributable to the problems as mentioned in the foregoing, the following significant facts have been found.
More specifically, if the inner coating film to be applied meets the following two requirements, (1) the coating film having a specific resistance of at least 0.2 ohm.cm, and (2) the coating film having a surface roughness of at least 6 microns, then a spark current occurring at the time of a short circuit is limited to 200 .ANG. or lower, even when the resistance value between the electron gun 7 and the high-voltage conductor 4 is of a relatively low value of at least about 2 K.OMEGA.. The foregoing underlies the present invention.